Foamed articles are in general light in weight and excellent in heat-insulating property and energy absorption property against external stress such as impact force, and therefore, are widely used for various applications such as heat-insulating materials, cushion materials for external stress, core materials and food containers. Also as the starting materials for the foamed articles, there are various ones such as polystyrene resin, polyurethane resin and polyethylene resin.
Further as the starting materials for the foamed articles, employment of a polypropylene resin which is excellent in solvent resistance and advantageous from the viewpoint of cost and has a high melting point and a high modulus of elasticity, can be considered. However since the polypropylene resin is a crystalline resin, its melt viscosity and melt tensile strength are low. For that reason, strength of a cell wall formed when expanding under molten condition is not sufficient, and the cell wall is apt to be broken. Also when the temperature of the polypropylene resin in the molten state is decreased in order to make sufficient the strength of the cell wall, a melt viscosity is increased rapidly. Therefore it was very difficult to control the melt viscosity to the value suitable for expanding by adjusting a melting temperature of the polypropylene resin. For these reasons, it was difficult to obtain a foamed article, which has a high closed cell content, by using a conventional polypropylene resin.
As approaches to solve the problem of easily causing the breakage of the cell wall formed when expanding the polypropylene resin, there have been proposed, for example, a method of expanding a blend of polypropylene and polybutene-1 (JP-B-40167/1981), a method of expanding a blend of polypropylene and polyethylene (JP-B-59252/1982) and a method of crosslinking a polypropylene resin with a crosslinking agent (JP-A-254142/1988).
However, even with those methods, the above-mentioned problem of the cell walls being easily broken could not be sufficiently solved, and therefore, it was difficult to obtain foamed articles having a high expansion ratio and a high closed cell content. Also those foamed articles have a drawback of being difficult to be recycled because a large amount of resins other than the polypropylene resin is mixed therein or because the articles are difficult to melt due to intermolecular crosslinking.
Also in recent years, many methods of making a sheet-like foamed article by using, as a resin material, polypropylene having a structure of long branched chains (JP-A-506875/1993, JP-A-363227/1992, etc.). However these methods have drawbacks such that since the specific resin is used, starting materials therefor are expensive and thus the obtained foamed articles are less applicable to various uses.
In case where such a polypropylene resin is used, though there is seen an improvement of foamability property, the degree of the improvement is not enough. Further in case where a thick foamed article is made by using such a polypropylene resin, an effect of increasing the strength of the cell wall when cooled from outside thereof is decreased depending on the increase of thickness. Therefore, it was difficult to obtain a foamed article having a thickness of not less than 5 mm and suitable properties as mentioned above.
An object of the present invention is to provide a foamed article comprising the modified polypropylene resin and having a high expansion ratio, high closed cell content and good external appearance.
Another object of the present invention is to provide a thick foamed article comprising the modified polypropylene resin and having a high expansion ratio, high closed cell content and good external appearance.